can I load, render the main page first using blazor server, then switch to a subpage and run it using webassembly? I know I can create webassembly + asp.net but then the main page is displayed using webassembly (seo unfriendly)
... but then the main page is displayed (seo unfriendly)
What you are looking for is called "server prerendering".
It requires hosting on asp.net (using the Webassembly hosted model).
See the official docs and also search the term for more walk-throughs and videos.
Here is a useful blog that links to a few more.